Field of the Invention
The invention relates to a sheet-transport drum in a sheet-processing machine. The drum includes at least two sheet-supporting surfaces with suction openings. Each respective sheet-supporting surface is formed of a first comb segment and a second comb segment. The comb segments have segment teeth with suction grooves formed therein. A trailing-edge suction groove is provided in the sheet-supporting surface of a respective second comb segment and a rotary valve provides a timed supply of suction air. The invention also relates to a sheet-fed printing machine, in particular a digital printing machine, including a sheet-transport drum with vacuum sources.
The use of digital printing machines to print shorts runs or customized printed images on sheets of paper, paperboard, and cardboard is known in the art. When inkjet heads are used to print on the sheets, a transport system moves a respective sheet underneath the inkjet heads at a minimum distance. Known transport systems are revolving transport belts, for instance embodied as suction belts, and rotating cylinders, also known as jetting cylinders, or revolving tables.
Machine concepts that use cylinders as described, for instance, in U.S. Patent Application Publication US 2009/0284561 A1, include a number of inkjet print heads disposed above a jetting cylinder to print on sheets that are moved past at a short distance from the print heads. A jetting cylinder may simultaneously hold a plurality of sheets by suction and transport them.
In order to guarantee a high printing quality and to avoid damage to the print heads, an important aspect is to ensure that a respective sheet rests securely on the jetting cylinder. If a sheet does not rest securely thereon and if dog ears stick out, for instance, they may damage the print heads. In order to prevent that, it is known to equip the printing machine with a detection device and to quickly stop the machine if there are any corners or edges that stick out. However, that means a considerable reduction of the throughput of the machine.
Sheet-transport drums including gripper systems for holding the sheets to be transported are known as devices for guiding sheets and are disclosed, for instance, in German Patent Application DE 42 21 046 A1, corresponding to U.S. Pat. No. 5,172,634, German Patent Application DE 101 02 226 A1, corresponding to U.S. Pat. No. 6,659,456, and European Patent Application EP 1 415 804 A1, corresponding to U.S. Pat. No. 7,150,456. The latter discloses a sheet-transport drum with sheet-supporting surfaces formed of two comb segments. In order to ensure that a sheet resting thereon is held securely, the comb segments have suction grooves that are supplied with suction air by using a rotary valve disposed inside the sheet-transport drum.
A disadvantage of such a sheet-transport drum is that the suction air breaks down if there is a sheet-supporting surface with no sheet resting thereon and the further sheets on the drum may no longer be held securely as a consequence. That is the case during the start-up and shutdown of the printing machine and when defective sheets are discharged before they reach the sheet-transport drum.